There is known a solid electrolytic capacitor comprising a layer of conductive polymer as solid electrolyte namely semiconductor layer, which is obtained by forming a dielectric oxide film in an anode body composed of a sintered body or a foil made of a valve action metal such as tantalum, niobium, aluminum, titanium or the like using an anodization method, and then forming the conductive polymer layer on the oxide film (Patent Document 1, etc.) This solid electrolytic capacitor has been used in various applications since it is possible to decrease ESR when compared with that of a solid electrolytic capacitor using manganese dioxide as solid electrolyte, which has conventionally been used.
In the preparation of a conductive polymer, a thiophene derivative, a pyrrole derivative, an aniline derivative or the like is usually used as a monomer. In order to use this conductive polymer as a solid electrolyte in a solid electrolytic capacitor, there is usually used a chemical oxidation polymerization method in which an oxidizing agent and a dopant are added to the monomer and the mixture is reacted on a dielectric oxide film to form a conductive polymer layer (Patent Document 2, etc.), or an electrolytic polymerization method in which a dopant is added to the monomer and the mixture is reacted on a dielectric oxide film by application of electricity to form a conductive polymer layer (Patent Document 3, etc.) There is also used a method in which a solution of a conductive polymer is prepared and an anode body with a dielectric oxide film formed thereon is impregnated with the solution and then the solution is converted into a coating film by drying to form a conductive polymer layer (coating method).
By the way, when electronic components such as a solid electrolytic capacitor are mounted on a circuit board, reflow soldering is carried out. Reflow soldering is a soldering method in which a solder is preliminarily supplied to the position to be connected with electronic components on a circuit board and electronic components are placed thereon, followed by heating to 200° C. or higher using a reflow furnace.
In a solid electrolytic capacitor comprising an anode body composed of a foil of aluminum or the like, ESR scarcely varies even by heating at the time of reflow soldering and thus it is comparatively easy to design an electronic equipment.
However, in a solid electrolytic capacitor comprising an anode body composed of a sintered body of tantalum or the like, ESR as important characteristics of a capacitor has sometimes increased largely when compared with that before heating by heating at the time of this reflow soldering. Also, a further increase in ESR has arisen by electrically repeated use of the capacitor after ESR once increased
Since the recently used solid electrolytic capacitor exhibits a small value of ESR, such as several mΩ, even slight increase by about 1 mΩ is a problem which cannot be omitted in the design of the electronic equipment.
Patent Document 4 proposes, as a solid electrolytic capacitor in which an increase in ESR due to high temperature and high humidity is suppressed, a solid electrolytic capacitor comprising an anode body composed of a valve action metal or an alloy containing a valve action metal as a main component, a dielectric layer formed by anodizing the anode body, an electrolyte layer formed on the dielectric layer and a cathode formed on the electrolyte layer, in which the cathode comprises a silver layer formed using a silver paste, wherein the silver layer contains 0.01 to 0.5% by weight of sulfur and/or a sulfur compound. In this solid electrolytic capacitor, migration of silver contained in the silver layer is prevented by the addition of sulfur or the like and, even after standing in high temperature and high humidity at 85° C. and 85% RH, ESR may scarcely increase. However, this solid electrolytic capacitor exhibits ESR as large as over 10 mΩ. Also, an increase in ESR due to reflow heat at about 260° C. is not sufficiently suppressed.
Patent Document 5 proposes that a boron simple substance or a boron-based intermetallic compound powder is added to a silver layer thereby preventing migration of silver contained in the silver layer. However, there is a problem that, even in case the boron simple substance or boron-based intermetallic compound powder is added to the silver layer, migration of silver contained in the silver layer cannot be sufficiently prevented and short circuit still arises in the solid electrolytic capacitor under an environment of high temperature and high humidity, and thus a leakage current increases.